Starting from Poincare group, one can obtain Carroll group as a contraction by taking the speed of light going to zero limit. This was first introduced by Levy-Leblond [1]} and Sen Gupta [2]} as a “degenerate cousin of the Poincare group".
Numerous counter-intuitive things occur in this limit. The light-cone closes completely, the space-time metric becomes degenerate and new non-Lorentzian structures emerge on the spacetime manifold [3]}. These Carrollian manifolds can be understood mathematically as fibre-bundles where there are two distinct metrics on the base and the fibre instead of a single non-degenerate metric for the entire spacetime. Interestingly, any null hypersurface is a Carroll manifold. Two of the most important examples of null hypersurfaces are null boundary of asymptotically flat spacetimes and event horizons of generic black holes. Thus Carrollian and conformal Carrollian structures are inexorably linked with scatterings in flat spacetimes and black hole physics.
